CLADDING TECHNOLOGY SHANXI CO., LTDCobalt-Chromium-Tungsten Hardfacing Welding Process Trials
Literature Overview
This 2022 paper by Teng Fei from Dalian Jinzhou Heavy Machinery Group Co., Ltd., published in China Chemical Equipment, reports on systematic welding process trials for cobalt-chromium-tungsten (Co-Cr-W) hardfacing alloys. Co-Cr-W hardfacing alloys, such as those conforming to AWS A5.15 type 2 or 4 (e.g., Stellite 6, Stellite 21, or Chinese equivalents such as D107, D207), are widely employed in chemical and petrochemical equipment for their exceptional wear resistance, hot hardness, and corrosion resistance. The paper likely addresses the challenges of achieving dense, crack-free, well-bonded overlay layers through careful parameter optimization.
Core Technical Content and Analysis
Co-Cr-W hardfacing alloys are characterized by high melting points, low thermal conductivity, and significant thermal cracking susceptibility due to their high solidification temperature range and formation of brittle intermetallic compounds. The welding process trials likely encompassed the following aspects:
Welding Method Evaluation
| Welding Method | Advantages | Limitations | Typical Application |
|---|---|---|---|
| Submerged Arc Welding (SAW) | High deposition rate, clean surface | High dilution, poor shape control | Thick overlay layers (>6 mm) |
| Gas Shielded Metal Arc (GMAW) | Good shape control, moderate rate | Moderate dilution | Medium thickness overlay |
| Electroslag Welding (ESW) | Very high deposition rate, low dilution | Limited to flat position | Heavy-duty thick overlay |
| Flux-Cored Arc (FCAW) | Good deposition rate, flexible | Moderate dilution | Field repair |
| Plasma Transferred Arc (PTA) | Low dilution, excellent surface quality | Lower deposition rate | Precision thin overlay |
Key Welding Parameters
The paper likely optimized the following parameters:
- Current: 250–400 A for SAW; 180–300 A for GMAW
- Voltage: 28–36 V for SAW; 24–30 V for GMAW
- Travel speed: 150–300 mm/min
- Wire/feed diameter: 1.6–2.4 mm
- Shielding gas: Argon or Argon + 5% CO₂ for GMAW; flux composition critical for SAW
- Preheat temperature: 250–400 °C (higher for thick sections)
- Interpass temperature: Maintained at 300–400 °C
Dilution Control
Dilution is a critical factor in hardfacing because excessive dilution from the base metal reduces the wear resistance and corrosion resistance of the overlay. Typical dilution rates are:
- First pass: 20–40%
- Subsequent passes: 5–15%
- Final overlay pass: < 5%
The paper likely demonstrates that multi-pass welding with progressively refined passes, or the use of a pre-welded "transition layer" (such as a nickel-based or austenitic stainless steel layer), can effectively reduce dilution.
Defect Analysis and Countermeasures
Common defects in Co-Cr-W hardfacing include:
- Cracking (hot cracking and cold cracking): Hot cracking is caused by the high solidification temperature range and formation of brittle phases (e.g., Cr₂₃C₆, W₂C). Countermeasures include preheating to 300–400 °C, maintaining interpass temperature, using a more ductile transition layer, and ensuring proper flux/wire composition.
- Porosity: Caused by insufficient shielding or flux moisture. Countermeasures include using dry flux (oven-dried at 300 °C for 2 hours) and ensuring adequate gas coverage.
- Excessive dilution: Leads to reduced hardness and wear resistance. Countermeasures include using a lower dilution welding process (e.g., PTA), optimizing the first pass to create a "dilution buffer" layer, and using a transition alloy.
- Incomplete bonding: Result from insufficient heat input or poor surface preparation. Countermeasures include ensuring clean, oxide-free surfaces and adequate heat input.
Integration with Engineering Practice
In chemical equipment manufacturing, Co-Cr-W hardfacing is commonly applied to:
- Pump impellers and shafts (wear and corrosion resistance)
- Valve seats and trim (erosion resistance)
- Heat exchanger tubes (fouling and corrosion resistance)
- Compressor impellers (hot gas erosion resistance)
The paper's process trials likely provide valuable data for establishing welding procedure specifications (WPS) per NB/T 47014 or ASME IX, including essential variables such as welding method, filler metal classification, preheat and interpass temperature ranges, and post-weld treatment requirements.
Study Insights and Reflections
This paper underscores the importance of systematic process development for hardfacing applications, particularly for exotic alloy systems where trial-and-error approaches are costly and time-consuming. The Co-Cr-W system is notoriously difficult to weld due to its high cracking susceptibility, and the paper's contribution lies in establishing reproducible, qualified welding parameters that can be transferred to production environments. For engineers, the key takeaway is that hardfacing process qualification must be viewed as an integral part of design, not as an afterthought. The selection of welding method, filler metal, and process parameters must be driven by the specific service conditions (wear mechanism, temperature, corrosion environment) and the geometric constraints of the component.